AC Theory: Resonance and Filters
A low-pass filter
Attenuates high frequency energy but passes low frequency energy
high pass filter
Attenuates low-frequency energy but passes high-frequency energy
C is the symbol for
a capacitor
L is the symbol for
an inductor
In a series ac circuit, inductive reactance leads by 90°, compared with the zero reference angle of the resistance,
and capacitive reactance lags by 90°.
You may choose to use a series resisitance instead of a choke
because it is a cheaper and smaller filter.
The symbol for resonant frequency is
fr
fc is the symbol for
frequency cutoff
A narrower BW signifies
higher selectivity
The high-pass filter passes to the load all frequencies ...
higher than the cutoff frequency: fc.
The width of the resonant band of frequencies centered around fr
is called the bandwidth of the tuned circuit.
If dB value is negative,
it indicates a loss and is referred to as an attenuation.
If dB value is positive,
it indicates an increase in power.
A wider BW signifies
lower selectivity.
The low-pass filter passes to the load all frquencies
lower than the cutoff frequency: fc.
Output power equals to half of the
max output power at cutoff frequency
The higher the ratio of the reactance at resonance to the series resistance,
the higher the Q and the sharper the resonance effect.
At the resonant frequency,
the inductive reactance and capacitive reactance are equal.
At cutoff frequency fc,
the output voltage or current is 70.7% of its maximum value
3dB equals
½ power
attenuation
The ability of a filter to reduce the amplitude of undesired frequencies is called ....